1. Field of the Invention
This invention relates generally to a flat type cathode ray tube and is directed more particularly to a flat type cathode ray tube which includes a funnel with its envelope having a configuration to improve the reliability thereof.
2. Description of the Prior Art
A prior art flat type cathode ray tube including a flat envelope which consists of a panel and a funnel and an electron gun mounted therein will be now described with reference to FIGS. 1 to 3. Especially, one of the methods of making the funnel thereof will be explained in which a glass plate is heated to be softened and then a cope or upper mold is moved down to the soft glass plate from the upper side thereof to carry out the molding thereof. For example, as shown in FIG. 1, a fixed lower mold or drag 71 having a concave portion 70 is provided. A glass plate 72 to be molded is located thereon to cover the portion 70, a guide wall 73 is disposed thereon, and an upper mold or cope 74 is inserted into the guide wall 73 to be slidably guided. In this case, the cope 74 has an outer configuration corresponding to the inner configuration of the funnel to be finally made. Then, the drag 71, cope 74, guide wall 73 and glass plate 72 are all heated to soften the glass plate 72. Thus, as shown in FIG. 2, by the weight of the glass plate 72 and cope 74 themselves or by the urging force of the cope 74, the glass plate 72 is deformed to be a molded plate or a funnel 1b having the configuration corresponding to that of the cope 74.
The funnel 1b molded as above has a flat plate portion 1b.sub.1, a peripheral wall portion 1b.sub.2 thereof and a flange portion 1b.sub.3 extended outwards therefrom as shown in FIG. 3. In this case, as the flange portion 1b.sub.3 is formed of the glass plate portion gripped by the drag 71 and the guide wall 73 as shown in FIGS. 1 and 2, the peripheral portion of the flange 1b.sub.3 is flattened in shape. Therefore, as indicated by a one-dot chain line d in FIG. 3, the unnecessary portion thereof is cut away. Then, the end face of the flange 1b.sub.3 is lapped such that a depth D and height H of the funnel 1b each become a predetermined value, and that a flat and smooth end surface 1b.sub.4 is provided. The end surface 1b.sub.4 thus made becomes a surface to be frit-fitted to the panel (not shown). In this case, however, a fine flaw is easily caused in the end surface 1b.sub.4 upon the lapping work. If the panel is frit-fitted to the end surface 1b.sub.4 of the funnel 1b thus made, the above-mentioned fine flaw will become a cause to generate a crack upon heat treating. As a result, the inside of the envelope thus manufactured cannot be kept in high vacuum or an explosion will be caused to lower the reliability of the envelope.
The funnel 1b thus made has the flat plate portion 1b.sub.1 with the thickness substantially equal to that of the initial glass plate 72, for example, 2.8 mm. However, since the portion of the glass plate 72 corresponding to the peripheral wall portion 1b.sub.2 and flange portion 1b.sub.3 of the funnel 1b are extended to form the flat plate portion 1b.sub.1, the thickness of the portions 1b.sub.2 and 1b.sub.3 are reduced as compared with that of the flat plate portion 1b.sub.1 and in addition, scattered, which is substantial especially at the flange portion 1b.sub.3. Further, a bent portion 1b.sub.8 from the peripheral wall portion 1b.sub.2 to the flange portion 1b.sub.3 becomes too thin in thickness and can not present sufficient mechanical strength.